Process for the treatment of melted magnesium and magnesium alloys



Patented Mar. 31, 1953 PROCESS FOR THE TREATMENT .OF-MELT-ED MAGNESIUM AND MAGNESIUM ALLOYS Sven Christian Pyk, Nyna'shamn, Sweden No'Drawing- Application March 4, 1949, Serial No.79,725; In 'SwedenMarch.11,-1948 2 Claims.

It is well-known that the presence of iron,

"nickel 'and'cobalt in magnesium and magnesium jalloys, even in very 'small quantities, causes an appreciable deterioration'in the resistance to cor- 'rosion.

In pure magnesium, for example, the rate of corrosion increases extremely rapidly when the iron content rises above 0.016%. Thus, with 0.020% it will be about five hundred times greater in a 3% salt solution than with 0.016% iron in the metal.

For magnesium-aluminium-zinc-manganese alloys this transitional value lies at about 0.002

Thus, it is very important in many cases to have magnesium which is free from iron available as the initial material. With thermal methods of production in which magnesium is obtained in the form of crystals which ar formed from magnesium vapour, or where the magnesium is produced by electrolytic fusion methods and purified by distillation the metal fulfils these requirements for practical purposes, since in th s case the iron content is less than 0.001%.

The re-meltin and alloying or other treatment of melted magnesium as in the cas of production by electrolysis is preferably carried out in apparatus consisting of iron or iron alloys free from nickel. In this respect the most suitable apparatus appears to be an apparatus of austenitic steel with a high manganese and high carbon content, but even in this case it is not possible to prevent the absorption of iron entirely whilst in certain cases the presence of manganese may be accompanied by disadvantages. A certain absorption of iron always occurs however. Thus, in magnesium of the ordinary commercial type the iron content usually amounts to about 0.03%.

When overheating the molten magnesium alloys for the purpose of obtaining a fine grain, or when introducing alloying substances which require a high temperature, the absorption of iron will increase. On introducin zirconium as an alloying constituent, for example, this iron absorption is particularly troublesome as dinicultly fusible zirconium-iron compounds occur in the melt which even when they are partially removed together with the iron, nevertheless increase the amount of zirconium employed.

It is already known that the iron absorption can be limited by aluminizing the crucible in which the metal is treated. As aluminium is soluble in magnesium, however, the aluminium layer in the crucible is rapidly dissolved so that at the outset magnesium containing aluminium 'isproduced, this being followed "by'ircn absorption. In some cases it is also desirable to obtain a product free from aluminium, this being the case, for example, in magnesium alloys containing' zirconium.

In other Processes crucibles'of chromium-iron are employed which are free from nickel and have a high chromium content up to 28%. In consequence of the greater resistance to oxidation thereby obtained with a consequent reduction in the scaling of the crucible material the rate of solution of the iron is reduced without its being possible however, to prevent the absorption of iron in a satisfactory manner. Furthermore, crucibles of this kind are unsuitable on account of the fact that they become brittle in the course of time and there is also a danger of crack formation.

It has now been surprisingly confirmed that the iron absorption can be prevented entirely Whilst at the same time the brittleness due to the chromium with the consequent risk of cracks can be avoided, thereby allowing magnesium which is substantially free from iron to be obtained, if the treatment of the magnesium or magnesium alloys is carried out in an apparatus, crucibles or the like, which at least as far as the part or parts which come into contact with the magnesium or magnesium alloys in a molten condition are concerned, is provided with a protectiv layer of chromium free from iron. This protective layer may be produced in a known manner for example, by the precipitation of chromium electrolytically or by chemical means by treating the apparatus with a gaseous mixture of chromium chloride and hydrogen or with chromium carbonyl.

It is also found that when re-melting the ma nesium in iron crucibles having a protective layer according to the invention and even after a long period of use after the chromium layer has flaked off, for example, in connection with the cleaning of the crucible or by oxidation, the action remains unaffected on account of the fact that while the crucible is in use chromium has been diffused into the outer surface of the iron crucible to a sufficient depth. The effect produced by the protective layer of chromium according to the invention can be increased in accordance with the latter by the heat treatment of the parts of the apparatus or crucible in question in such a way that at a raised temperature the diffusion of the chromium is increased before the apparatus or crucible is taken into service for the required purpose.

By way of example it may be mentioned that crystallized magnesium obtained by the condensation of magnesium vapour and containing 0.000392 iron can be remelted in an electrolytically chromated crucible of iron. Only after a large number of melts have been carried out in the crucible was it possible to notice a certain though slight increase of the iron content in the remelted metal. Not more than 0.0007% iron could be traced in the metal. Furthermore, the

metal was free from chromium and irom the point of view of corrosion was equal to magnesium purified by distillation. Magnesium remelted in an iron crucible of this kind which had not been treated according to the invention was found to contain 0.25% iron.

I claim:

1. In the art of handling pure molten magnesium without contamination thereof, the process which comprises melting pure magnesium in a .ferrous metal crucible having all parts thereof which come into contact with the molten magnesium covered with a heat treated layer of metal .lic chromium free from iron precipitated on the ferrous metal in direct contact with and partially difiused by heat treatment into the under- '4 lying metal, whereby contamination of the molten magnesium with iron is prevented.

2. In the art of handling pure molten magnesium without contamination thereof, the process which comprises melting pure magnesium in a ferrous metal crucible having all parts thereof which come into contact with the molten magnesium electroplated with a heat treated layer of metallic chromium free from iron and partially diffused by heat treatment into the underlying metal, whereby contamination of the molten magnesium with iron is prevented.

SVEN CHRISTIAN PYK.

REFERENCES CITED The following references are of record in the file oi? this patent:

UNITED STATES PA'IEN'I'S 

1. IN THE ART OF HANDLING PURE MOLTEN MAGNESIUM WITHOUT CONTAMINATION THEREOF, THE PROCESS WHICH COMPRISES MELTING PURE MAGNERSIUM IN A FERROUS METAL CRUCIBLE HAVING ALL PARTS THEREOF WHICH COME INTO CONTACT WITH THE MOLTEN MAGNESIUM COVERED WITH A HEAT TREATED LAYER OF METALLIC CHROMIUM FREE FROM IRON PRECIPITATED ON THE FERROUS METAL IN DIRECT CONTACT WITH AND PARTIALLY DIFFUSED BY HEAT TREATMENT INTO THE UNDERLYING METAL, WHEREBY CONTAMINATION OF THE MOLTEN MAGNESIUM WITH IRON IS PREVENTED. 